The present invention relates to placement of stents within tubular structures of the body such as the veins or arteries of the vascular system; the airways of the respiratory system; and the passages of the intestinal tract. As referred to in this disclosure, stents are structures which are placed within tubular structures of the body so as to reinforce or line the tubular structure. For example, a stent may be placed within an artery to maintain patency of the artery after angioplasty or to reinforce the arterial wall as, for example, if an aneurysm is present. Also, stents may be placed within the esophagus or within an airway to keep the airway or esophagus open in the presence of a tumor or other abnormal growth. Stents can be placed within the body using a elongated probe such as a catheter or endoscope. The stent is held on the distal end of the probe. The stent typically is retained on the probe by a holding device such as a balloon which can be operated from the proximal end of the probe. The physician advances the probe into the tubular structure, distal end first, until the distal end of the probe is positioned at the appropriate location within the tubular structure. When the distal end of the probe is appropriately positioned, the physician actuates the holding device to release the stent. For example, in the case of a balloon actuated stent, the physician may momentarily inflate the balloon so as to expand the stent into engagement with the tubular structure and then deflate the balloon so as to release the stent from the probe.
Procedures of this nature typically have been performed using fluoroscopic or other images acquired during the procedure. Fluoroscopic imaging exposes both the patient and the physician to radiation. Moreover, many internal tubular structures such as arteries are not readily visualized using fluoroscopic imaging unless contrast media are employed. These add to the complexity of the procedure and, in some cases, present some additional risk to the patient. If the procedure is performed using tomographic x-ray imaging (commonly referred to "CT" or "CAT") or magnetic resonance imaging (MRI) during the procedures, the apparatus is occupied for the entire time required to perform the procedure. The need for such apparatus during the procedure adds to the cost of the procedure and limits the locations where the procedure can be performed. Other procedures for placement of stents have been guided by direct visual observation of the body by means of optical systems included in an endoscope used to place the stent. These techniques are best applied in relatively large structures.
Certain stents have been developed for reinforcing tubular structures which include branching elements as, for example, the Y-shaped intersections of arteries or the Y-shaped intersection of the trachea and the primary bronchi. These stents include a first reinforcing element having one longitudinal axis and one or more branch or secondary reinforcing elements having longitudinal axes transverse to the longitudinal axis of the first element. For example, in reinforcing a generally Y-shaped section of a branching arterial network, the stent used is also generally Y-shaped. Positioning of branching stents poses additional problems. Thus, in placing an ordinary stent, with only a single tubular structure having uniform properties around its longitudinal axis, there is no need to control the orientation of the stent in roll or rotation around the longitudinal axis. Although the longitudinal axis of the stent must be reasonably well-aligned with the longitudinal axis of the tubular structure, this alignment typically is maintained by the mechanical engagement of the stent or probe in the tubular structure itself. However, when a branching stent is installed in a branching structure, the orientation of the branching stent must be matched to the orientation of the branching structure. For example, is a generally Y-shaped stent is to be installed in a generally Y-shaped branching structure, the orientation of the stent must be controlled so that the branches point in the correct directions to fit within the branches of the artery, airway or other tubular structure. Thus, many procedures for placing such stents involve careful threading of guide wires through the branching structure to guide deployment of the shunt. Other types of stents may also require control of orientation.
Other medical procedures have been performed heretofore using position-monitoring equipment in which a magnetic, electromagnetic or other non-ionizing field is transmitted to or from the probe. As described, for example, in U.S. Pat. Nos. 5,558,091, 5,391,199; 5,443,489; and in PCT International Publication WO 96/05768, the disclosures of which are hereby incorporated by reference herein, the position, orientation or both of the distal end of a probe used for procedures such as surgery, cardiac monitoring or cardiac ablation can be determined by using one or more field transducers such as a Hall effect or magnetoresistive device, coil or other antenna carried on the probe, typically at or adjacent the distal end of the probe. One or more additional field transducers are disposed outside the body in an external frame of reference. The field transducers preferably are arranged to detect or transmit non-ionizing fields or field components such as a magnetic field, electromagnetic radiation or acoustical energy such as ultrasonic vibration. By transmitting the field between the external field transducers and the field transducers on the probe, characteristics of field transmission between these devices can be determined. The position and/or orientation of the sensor in the external frame of reference can then be deduced from these transmission characteristics. Because the field transducer of the probe allows determination of the position of the probe, such transducer is also referred to as a "position sensor".
As described, for example, in the aforementioned U.S. Pat. No. 5,558,091, the frame of reference of the external field transducers can be registered with the frame of reference of imaging data such as magnetic resonance imaging data, computerized axial tomographic data, or conventional x-ray image data and hence the position and orientation data derived from the system can be displayed as a representation of the probe superimposed on an image of the patient's body. The physician can use this information to guide the probe to the desired location within the patient's body, and to monitor its orientation during treatment or measurement of the body structure. This arrangement greatly enhances the ability of the physician to navigate the distal end of the probe through bodily structures. The transducer-based system avoids the difficulties associated with navigation of a probe by continuous imaging of the probe and patient during the procedure. For example, it avoids exposure to ionizing radiation inherent in fluoroscopic systems.